Composite products comprising a matrix metal and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like show great promise for a variety of applications because they combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite will show an improvement in such properties as strength, stiffness, contact wear resistance, and elevated temperature strength retention relative to the matrix metal in monolithic form, but the degree to which any given property may be improved depends largely on the specific constituents, their volume or weight fraction, and how they are processed in forming the composite. In some instances, the composite also may be lighter in weight than the matrix metal per se. Aluminum matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their higher stiffness, wear resistance and high temperature strength relative to aluminum.
Various metallurgical processes have been described for the fabrication of aluminum matrix composites, including methods based on powder metallurgy techniques and liquid-metal infiltration techniques which make use of pressure casting, vacuum casting, stirring, and wetting agents.
With powder metallurgy techniques, the metal in the form of a powder and the reinforcing material in the form of a powder, whiskers, chopped fibers, etc., are admixed and then either cold-pressed and sintered, or hot-pressed. The production of metal matrix composites by powder metallurgy techniques utilizing conventional processes imposes certain limitations with respect to the characteristics of the products attainable. The volume fraction of the ceramic phase in the composite is limited, in the case of particulates, typically to about 40 percent. Also, the pressing operation poses a limit on the practical size attainable. Only relatively simple product shapes are possible without subsequent processing (e.g., forming or machining) or without resorting to complex presses. Also, nonuniform shrinkage during sintering can occur, as well as nonuniformity of microstructure due to segregation in the compacts and grain growth.
U.S. Pat. No. 3,970,136 granted Jul. 20, 1976 to J. C. Cannell et al., describes a process for forming a metal matrix composite incorporating a fibrous reinforcement, e.g. silicon carbide or alumina whiskers, having a predetermined pattern of fiber orientation. The composite is made by placing parallel mats or felts of coplanar fibers in a mold with a reservoir of molten matrix metal, e.g., aluminum, between at least some of the mats, and applying pressure to force molten metal to penetrate the mats and surround the oriented fibers. Molten metal may be poured onto the stack of mats while being forced under pressure to flow between the mats. Loadings of up to about 50% by volume of reinforcing fibers in the composite have been reported.
The above-described infiltration process, in view of its dependence on outside pressure to force the molten matrix metal through the stack of fibrous mats, is subject to the vagaries of pressure-induced flow processes, i.e., possible non-uniformity of matrix formation, porosity, etc. Non-uniformity of properties is possible even though molten metal may be introduced at a multiplicity of sites within the fibrous array. Consequently, complicated mat/reservoir arrays and flow pathways need to be provided to achieve adequate and uniform penetration of the stack of fiber mats. Also, the aforesaid pressure-infiltration method allows for only a relatively low reinforcement to matrix volume fraction to be achieved because of the difficulty inherent in infiltrating a large mat volume. Still further, molds are required to contain the molten metal under pressure, which adds to the expense of the process. Finally, the aforesaid process, limited to infiltrating aligned particles or fibers, is not directed to formation of metal matrix composites reinforced with materials in the form of randomly oriented particles, whiskers or fibers.
In the fabrication of aluminum matrix-alumina filled composites, aluminum does not readily wet alumina, thereby making it difficult to form a coherent product. Other matrix metal-filler combinations are subject to the same considerations. Various solutions to this problem have been suggested. One such approach is to coat the alumina with a metal (e.g., nickel or tungsten), which is then hot-pressed along with the aluminum. In another technique, the aluminum is alloyed with lithium, and the alumina may be coated with silica. However, these composites exhibit variations in properties, or the coatings can degrade the filler, or the matrix contains lithium which can affect the matrix properties.
U.S. Pat. No. 4,232,091 to R. W. Grimshaw et al. overcomes certain difficulties in the art which are encountered in the production of aluminum matrix-alumina composites. This patent describes applying pressures of 75-375 kg/cm.sup.2 to force molten aluminum (or molten aluminum alloy) into a fibrous or whisker mat of alumina which has been preheated to 700.degree. to 1050.degree. C. The maximum volume ratio of alumina to metal in the resulting solid casting was 1/4. Because of its dependency on outside force to accomplish infiltration, this process is subject to many of the same deficiencies as that of Cannell et al.
European Patent Application Publication No. 115,742 describes making aluminum-alumina composites, especially useful as electrolytic cell components, by filling the voids of a preformed alumina matrix with molten aluminum. The application emphasizes the non-wettability of alumina by aluminum, and therefore various techniques are employed to wet the alumina throughout the preform. For example, the alumina is coated with a wetting agent of a diboride of titanium, zirconium, hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium, titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inert atmospheres, such as argon, are employed to facilitate wetting. This reference also shows applying pressure to cause molten aluminum to penetrate an uncoated matrix. In this aspect, infiltration is accomplished by evacuating the pores and then applying pressure to the molten aluminum in an inert atmosphere, e.g., argon. Alternatively, the preform can be infiltrated by vapor-phase aluminum deposition to wet the surface prior to filling the voids by infiltration with molten aluminum. To assure retention of the aluminum in the pores of the preform, heat treatment, e.g., at 1400.degree. to 1800.degree. C., in either a vacuum or in argon is required. Otherwise, either exposure of the pressure infiltrated material to gas or removal of the infiltration pressure will cause loss of aluminum from the body.
The use of wetting agents to effect infiltration of an alumina component in an electrolytic cell with molten metal is also shown in European Patent Application Publication No. 94353. This publication describes production of aluminum by electrowinning with a cell having a cathodic current feeder as a cell liner or substrate. In order to protect this substrate from molten cryolite, a thin coating of a mixture of a wetting agent and solubility suppressor is applied to the alumina substrate prior to start-up of the cell or while immersed in the molten aluminum produced by the electrolytic process. Wetting agents disclosed are titanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium, niobium, or calcium, and titanium is stated as the preferred agent. Compounds of boron, carbon and nitrogen are described as being useful in suppressing the solubility of the wetting agents in molten aluminum. The reference, however, does not suggest the production of metal matrix composites.
In addition to application of pressure and wetting agents, it has been disclosed that an applied vacuum will aid the penetration of molten aluminum into a porous ceramic compact. For example, U.S. Pat. No. 3,718,441 granted Feb. 27, 1973 to R. L. Landingham reports infiltration of a ceramic compact (e.g., boron carbide, alumina and beryllia) with either molten aluminum, beryllium, magnesium, titanium, vanadium, nickel or chromium under a vacuum of less than 10.sup.-6 torr. A vacuum of 10.sup.-2 to 10.sup.-6 torr resulted in poor wetting of the ceramic by the molten metal to the extent that the metal did not flow freely into the ceramic void spaces. However, wetting was said to have improved when the vacuum was reduced to less than 10.sup.-6 torr.
U.S. Pat. No. 3,864,154 granted Feb. 4, 1975 to G. E. Gazza et al. also shows the use of vacuum to achieve infiltration. The patent describes loading a cold-pressed compact of AlB.sub.12 powder onto a bed of cold-pressed aluminum powder. Additional aluminum was then positioned on top of the AlB.sub.12 powder compact. The crucible, loaded with the AlB.sub.12 compact "sandwiched" between the layers of aluminum powder, was placed in a vacuum furnace. The furnace was evacuated to approximately 10.sup.-5 torr to permit outgassing. The temperature was subsequently raised to 1100.degree. C. and maintained for a period of 3 hours. At these conditions, the molten aluminum penetrated the porous AlB.sub.12 compact.
A method for making composite materials containing a reinforcing material such as fibers, wires, powder, whiskers or the like is disclosed in European Patent Application Publication No. 045,002, published on Feb. 3, 1982 in the name of Donomoto. A composite material is produced by placing a porous reinforcing material (e.g., aligned fibers of alumina, carbon, or boron) that is non-reactive with the atmosphere and a molten metal (e.g., magnesium or aluminum) into a container having an open portion, blowing substantially pure oxygen into the container, then immersing the container in a pool of the molten metal whereby the molten metal infiltrates the interstices of the reinforcing material. The publication discloses that the molten metal reacts with the oxygen present in the container to form a solid oxidized form of the metal, creating a vacuum in the container which draws molten metal through the interstices of the reinforcing material and into the container. In an alternative embodiment, the publication discloses placing an oxygen getter element (e.g., magnesium) within the container to react with the oxygen in the container to create a vacuum which, with the assistance of 50 kg/cm.sup.2 argon pressurization of the molten metal, draws the molten metal (e.g., aluminum) into the container filled with reinforcing material (e.g., aligned carbon fibers).
U.S. Pat. No. 3,867,177 granted Feb. 18, 1975 to J. J. Ott et al. discloses a method for impregnating a porous body with a metal by first contacting the body with an "activator metal", then immersing the body in a "filler metal". Specifically, a porous mat or compacted body of filler material is immersed in a molten activator metal for a time sufficient to completely fill the interstices of the body with molten activator metal by the method of the Reding et al. U.S. Pat. No. 3,364,976, discussed below. Subsequently, upon solidification of the activator metal, the composite body is entirely immersed in a second metal and maintained for a time sufficient to allow the second metal to replace the activator metal to a desired extent. The formed body is then allowed to cool. It is also possible to at least partially remove the filler metal from within the porous body and replace it with at least a third metal, again by partially or totally immersing the porous body in a molten replacement metal for a sufficient time to dissolve or diffuse a desired amount of replacement metal into the porous body. The resultant body may also contain intermetallics of the metals in the interstices between the filler material. Utilizing a multiple step process, including the use of an activator metal to form a composite having a desired composition, is costly in both time and money. Further, the limitations on processing based on, e.g., compatibility of metals (i.e., solubility, melting point, reactivity, etc.), limit the ability to tailor the characteristics of the material for a desired purpose.
U.S. Pat. No. 3,529,655 granted Sep. 22, 1970 to G. D. Lawrence, discloses a process for forming composites of magnesium or magnesium alloys and silicon carbide whiskers. Specifically, a mold having at least one opening to the atmosphere and containing silicon carbide whiskers in the interior volume of the mold is immersed in a bath of molten magnesium so that all openings in the mold are below the surface of the molten magnesium for a time sufficient for the magnesium to fill the remaining volume of the mold cavity. It is said that as the molten metal enters the mold cavity it reacts with the air contained therein to form small amounts of magnesium oxide and magnesium nitride, thereby forming a vacuum which draws additional molten metal into the cavity and between the whiskers of silicon carbide. The filled mold is subsequently removed from the molten magnesium bath and the magnesium in the mold is allowed to solidify.
U.S. Pat. No. 3,364,976 granted Jan. 23, 1968 to John N. Reding et al. discloses creating a self-generated vacuum in a body to enhance penetration of a molten metal into the body. Specifically, a body, e.g., a graphite or steel mold, or a porous refractory material, is entirely submerged in a molten metal, e.g., magnesium, magnesium alloy or aluminum alloy. In the case of a mold, the mold cavity, which is filled with a gas, e.g., air, that is reactive with the molten metal, communicates with the externally located molten metal through at least one orifice in the mold. When the mold is submerged in the melt, filling of the cavity occurs as a vacuum is produced from the reaction between the gas in the cavity and the molten metal. Particularly, the vacuum is a result of the formation of a solid oxidized form of the metal.
U.S. Pat. No. 3,396,777 granted Aug. 13, 1968 to John N. Reding, Jr., discloses creating a self-generated vacuum to enhance penetration of a molten metal into a body of filler material. Specifically, the patent discloses a steel or iron container open to the atmosphere at one end, the container containing a particulate porous solid, e.g., coke or iron, and being covered at the open end with a lid having perforations or through-holes smaller in diameter than the particle size of the porous solid filler. The container also houses an atmosphere, e.g., air, within the porosity of the solid filler which is at least partially reactive with the molten metal, e.g., magnesium, aluminum, etc. The lid of the container is immersed a sufficient distance below the surface of the molten metal to prevent air from entering the container and the lid is held below the surface for a sufficient time for the atmosphere in the container to react with the molten metal to form a solid product. The reaction between the atmosphere and the molten metal results in a low pressure or substantial vacuum within the container and porous solid that draws the molten metal into the container and the pores of the porous solid.
The Reding, Jr., process is somewhat related to the processes disclosed by European Publication No. 045,002, and U.S. Pat. Nos., 3,867,177, 3,529,655, and 3,364,976, all of which were discussed above herein. Specifically, this Reding, Jr., Patent provides a bath of molten metal into which a container, containing a filler material therein, is immersed deeply enough to induce a reaction between gas in the cavity and the molten metal and to seal the cavity with the molten metal. In another aspect of this Patent, the surface of the molten bath of matrix metal, which may be subject to oxidation in the molten state when in contact with the ambient air, is covered with a protective layer or flux. The flux is swept aside when the container is introduced to the molten metal, but contaminants from the flux may nevertheless be incorporated into the bath of molten matrix metal and/or into the container and porous solid material to be infiltrated. Such contamination, even at very low levels, may be detrimental to the formation of the vacuum in the container, as well as to the physical properties of the resultant composite. Further, when the container is removed from the bath of molten matrix metal and excess matrix metal is drained from the container, loss of matrix metal from the infiltrated body can occur due to gravitational forces.
Accordingly, there has been a long felt need for a simple and reliable process for producing metal matrix composites that does not rely upon the use of externally applied pressure or vacuum, damaging wetting agents or the use of a pool of molten matrix metal, with their attendant disadvantages as noted above. In addition, there has been a long felt need for a process that minimizes the final machining operations needed to produce a metal matrix composite body. The present invention satisfies these and other needs by providing a process involving a self-generated vacuum for infiltrating a material (e.g., a ceramic material), which can be formed into a preform, with a molten matrix metal (e.g., aluminum, magnesium, bronze, copper, cast iron, etc.) in the presence of a reactive atmosphere (e.g., air, nitrogen, oxygen, etc.) under normal atmospheric pressures.